Method and apparatus for finish grinding piezoelectric crystals



Oct. 16, 1945. H. F. FRUTH 2,387,140

METHOD AND APPARATUS FOR FINISH GRINDNG PIEZOELECTRIC'CRYSTALS Filed April 29, 1944 f//// INVENTOR.

Patented Oct. 16, 19.45

METHOD AND APPARATUS FOR FINISH y orrlcs GRINDING PIEZOELECTRIC CRYSTALS Hal F. Fruth, Chicago, Ill., assignor to Galvin Manufacturing Corporation,

corporation of Illinois Application April 29, 1944,A Serial No. 533,278

8 Claims.

The present invention relates to improved methods and apparatus for grinding piezoelectric crystals such, for example, as quartz crystals, to raise the resonant frequencies of the crystals to a predetermined desired frequency.

It is an object of the present invention to provide improved methods and apparatus for easily,

rapidly and accurately face grind piezoelectric crystals to increase the resonant frequencies thereof at a predictable rate for each increment of grinding time.

It is another object of the invention to provide improved methods and apparatusk for random grinding the faces of a batch of piezoelectric crystals to produce a substantially uniform and predictable increase in theresonant frequencies of the crystals during each increment of grinding time.

According to another object of the invention, low contact pressures are at all time maintained Chicago, Ill., a

supporting member I2 by means of flexible suspension strings I3, Ita, I 3b, l3c and i3d, so that the bottom wall Illb oi' the container is normally disposed in a substantially horizontal plane. More in detail. the iive identified strings are suit ably tied together at their ends I3e, the upper end of the main supporting string I3 is tied to an between .the crystal faces and the abrasive material, thereby to prevent scratching of the crystal faces as the grinding action proceeds and to insure the production of crystals having highly polished face surfaces.

According to another and more specific object of the invention, the desired crystal face grinding is obtained by oscillating a mass of crystals and loose abrasive material to and rolling movement therebetween.

In accordance with a still further object of the invention, gravity forces acting upon the crystals are primarily relied upon to determine the contact pressures between the crystal faces and the loose abrasive material, and the faces contacting the abrasive material are intermittently changed by intermittently throwing the crystals away from the surface which supports the crystals and abrasive material. Y

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be under- -stood by reference to the following specification taken in connection ing. in which:

Fig. 1 is a side view, partially in section, illustrating improved apparatus for practicing the present improved method of crystal face grinding; and

Fig. 21s a top in Fig. 1.

Referring how to the drawing,

with the accompanying drawplan view of the apparatus shown the crystal grinding apparatus there illustrated comprises an open-top container indicated generally at Ill which is supported from a horizontally disposed produce relative sliding eyelet I2a anchored in the supporting member I2, and the free ends of the four strings i3d, I3b, I3c and i3d are suitably anchored in the respective corners dened by the four intersecting side panels I0a of the container I0. With this suspension arrangement, thevcontainer Ill is supported for movement in an arcuate path having the eyelet I2a as a center.

For the purpose of oscillating the container I0 back and forth over this arcuate path, driving means are provided which include an electric motor 20 having a rotor shaft 23 which is crankconnected to one side panel I0a of the container I0 by means of a crank wheel 24 and an arm 25. More specifically, the wheel 24 is mounted for rotation with the rotor shaft 23 and lcarries adjacent its peripheral edge a pivot pin 26 which is journaled within an opening through one end of the arm 25. At itsopposite end-the arm 25 carries a transverse pivot pin I9 which` is journaled within openings provided in the projecting parts of two L-brackets I8a and lBb riveted to one side panel Ilia of the container I0. The motor 2li may be of any desired slow speed type, but preferably is of the universal or direct current type adapted for rheostatic speed control, in

order that the rate of oscillation of the container I0 about the pivot point I2a may be adjusted as desired.

With the container I Il suspended from the supporting member l2 in the manner explained above, the upper surface of the bottom panni IIlb thereof is normally disposed in a substantially horizontal plane, such that it may` be used inthe manner more fully pointed out below to support a layer of loose abrasive materiali which is utilized in the face grinding of a.l batch of crystals 32. For the purpose of reducing relative movement between the individual abrasive particles making up the layer of abrasive material 3l. and relative sliding movement between the abrasive mass and the bottom panel Ib. thereby to enhance relative sliding movement between the crystal faces and the abrasive material, a screen II is provided within .the container I0 to cover the upper surface of the bottom panel Illb and thus provide a rough surface on which the abrasive material is supported. 'I'his screen is preferbottom wall ilb by means of four angle brackets ably of a very line mesh andissuitably soldered or otherwise rigidly secured to the upper surface of the panel Ich around its peripheral edges adjacent the side panels of the container.

The composite wall thus formed at the bottom of the container Il and comprised of the panel ilb and the screen I i is resilient in character and hence may be flexed in a direction normal to the supporting surface thereof for the purpose of throwing the crystals I2 away from the supportin surface in order to reverse the faces thereof which are disposed downwardly against the abrasive material li. Flexing of the two identified bottom parts ofthe container Il is obtained through the action of tapping solenoid i4 which includes an operating winding arranged for energization from a suitable current source indicated by the bracketed terminals 21. The operatingcircuit for this solenoid also includes the contacts of an on-oi! switch I and a commutating device which comprises a non-conductive disc 2l driven at a constant speed by a motor, not shown, and having its peripheral surface engaged by a wiper The solenoid Il is supported centrally of the Ila, ilb, iIc and Ild which are rivet connected to the bottom panel Nb of the container at their respective outwardly disposed ends. The inner ends of the identified brackets il converge centrally of the bottom panel Nb, and are screw connected to the lower part Ila of the solenoid winding spool. The upper end of the solenoid il is anchored against movement transversely of the container bottom panel ich by'means of anchor arms Ila. IIb, i1c and i'ld having outer ends rivet connected to the angle brackets ita, Hb, Ibo and lid, respectively, and inner ends screw connected to the upper solenoid spool part Ilb. 'Ihe solenoid it additionally comprises a plunger Il formed of magnetic material and carrying at its upper end a flanged striker part Md which is formed of non-magnetic material, such, for example, as brass, and is provided with a flange or head overlying the opening through the spool part lib. This head is adapted to impact or strike the under surface of the bottom panel IOb with hammer like effect each time the operating winding of the solenoid Il is energized.

In utilizing the above described apparatus to practice the present improved grinding methods, a mass of loose granular abrasive material is deposited within the container il to be supported by the screen the motor 2l is initiated for the purpose of oscillating the container i0 back and forth through the arcuate path, the length of which is determined by the radial distance of the pivot pin from the center of the rotor shaft 23. Thus during each revolution of the wheel 24, the container i0 is moved from the illustrated horizontal position thereof to one limit of the described arcuate path, back to the opposite limit of the path, and then is returned to the illustrated starting'position thereof. As the ,container Il is moved in either direction away from its illustrated horizontal position the screen ii and bottom panel ith are obviously tilted relative to the horizontal, the direction of tilt being reversed each time the container is moved through its horizontal position. Thus, it will be apparent that concurrently with the oscillatory motion of the container. the upper supporting surface of its bottom parts are alternately tilted first in one direction and then in the opposite direction relative to the restraint against ii, following which operation of 4 surface of the screen horizontal. Thev initial oscillatory movement of the container i0 is utilized to spread -the loose abrasive material over the screen il so that the depth of the abrasive materialbecomes reasonably uniform throughout the upper surface area of the screen. After this leveling of the abrasive material is completed, operation of the motor 20 to oscillate the container i0 may be stopped for the purpose of permitting the piezoelectric quartz crystals to be disposed surface of the abrasive material.

In this regard it may be pointed out that before the finish grinding operation about to be described is started, the quartz crystals are first cut from the crystal stock and are then machine ground with high, pressure grinding apparatus ,y

to rectangular dimensions slightly greater than those desired. After the crystal wafers have been deposited upon the surface of the abrasive material 3i within the container I0, operation of the motor 2B may be resumed in order to reinitiate oscillatory movement'of the container back and forth along the described arcuate path. During such oscillatory movement and due to the tilting of the abrasive surface which occurs incident thereto, relative sliding movement of a completely random character is produced between the abrasive material and the faces of the crystals supported upon this material. This relative sliding movement is also caused by the difference between the relative masses of the crystal blanks and the individual abrasive particles. Thus, on a comparative basis each abrasive granule is much lighter or has less mass than a single crystal wafer. Moreover, the abrasive granules tend to pack, such that the relative movement therebetween is small as compared with the lrelative sliding movement between the abrasive particles and the contacting faces of the crystals. This displacement of the abrasive granules relative to each other is to some extent at least, enhanced by the action of the rough Ii to anchor those particles which engage the screen surfaces against movement relative to this surface. Because of the described factors, the crystals are slid back and forth over the upper strata of abrasive particles so that the downwardly disposed faces thereof are rapidly and evenly abraded. In this'regard, it is pointed out that the contact pressure between each downwardly disposed crystal face and the abrasive particles is determined solely by the magnitude of the gravitational forces acting lupon the crystal and hence by the mass of the crystal.

Such contact pressures are exceedingly small, never exceeding a value of approximately five grams for the total face area of the heaviest crystal in use. Thus any tendency to produce scratches in the faces of the crystals during the abrading operations is entirely eliminated, with the result that highly polished unmarred face surfaces are produced. Also, recurrent movement of a' single abrasive particle along a specific path across a particular crystal face is positively prevented. Accordingly, scratches ln the crystal faces are wholly obviated.

As previously indicated, the solenoid il is utilized intermittently to from the screen ii for the purpose of changing the faces thereof which are disposed downwardly in engagement with the abrasive material. To initiate this action concurrently with the start of the oscillatory movement of the container il, the switch l0 is closed to prepare a circuit for energizing the operating winding of .the solenoid face downwardly over the all tendencies to form deepk throw the crystals t2 away Il at the same time that the circuit for energizing-the motor 2g is completed. With the switch It closed and the constantly rotated commutating disc 2l in operation, the circuit 'for energizing l underside of the bottom panel lob by the plunger part Md, those crystals 32 which are disposed centrally of the screen Il are flung or thrown upwardly so that at least a part thereof are moved end over end, with the result that the opposite faces thereof are disposed' downwardly to engage the abrasive material Il when the return or fallacting uponthe crystals disposed radially away lar speed of container oscillation, may be ie-.- termined on an entirely accurate empirical basis from carefully observed test runs. With this data available, the crystals of a selected batch made, 'during the primary grinding step. be ground for an interval carefully selected to Pfoui overgrlnding of any of the crystals in the After the primary grinding of the crystals for the selected time interval is completed, the crystals are Ilgl'eilated from the grinding mixture.

" cleaned. measured to determine the resonant frequencies thereof, and grouped or classified according to their measured resonant frequencies. In the usual case. certain of the crystals in the batch will be found to have been ground to resonant frequencies which are so close to the desired predetermined frequency es to require no Y ing movement thereof is completed. The forcesfrom the center of the screen H toward the edges ofthe screen may be insufficient to effect the upward movement of these crystals which is required to cause the same -to turn over. Since, however. the striking action against the underside of the bottom panel iilb is intermittently repeated, the opposite faces of all crystals will at one time or another be disposed downwardly to engage the abrasive material. On an average basis over an extended grinding interval, substantially uniform grinding of both faces of each crystal is obtained, due to the crystal position changing action of the solenoid Il. The periodic impacting of the bottom panel lub also serves to shake up the loose granular abrasive material and then preclude any possibility of this material becoming so tightly packed as to detract from the desired rapid abrasion. Also, the shaking up of the abrasive material insures -that over an extended grinding period substantially all of the abrasive granules will be substantially uniformly utilized in actual contact with the crystal surfaces. As a result, the abrading action as measured in -terms of crystal frequency change per unit of grinding time is rendered substantially f more uniform.

More specifically considered, the finish grinding of the faces of a given batch of quartz crystal is preferably carried out in two or more steps. During the ilrst or primary step, the crystals are ground to approach the particular resonant frequency desired for each crystal. In this regard it will be understood that the crystal blanks as initially cut are lap ground to such dimensions that the resonant frequency of eachcrystal is well below the particular resonant frequency which isdesired. During the primary grinding step, therefore, the extent of grinding is limited not to exceed an amount which will bring any of the crystals to a resonant frequency higher than that desired. It has been found that the primary grinding step may be satisfactorilyrc/arried out by using a charge of abrasive material 3i which consists ofsilicon carbide of any mesh ranging from 180 to 250. 4In this regard it will be understood that the extent of crystal grinding, as measured in terms of frequency change of crystals of a particular size per unit of grinding time when using an abrasive of a particular character and size and a particufurther grinding. 'I'he remaining crystals, i. e. those having resonant frequencies well below the desired value, are subjected to further grinding To this end, the non-acceptable crystals are returned to the container il and the container is oscillated for a second predetermined time interval calculated to increase the resonant frequencies of at least a portion of the group to the desiredfrequency. This step by step process of grinding, cleaning and testing is repeated until the predominate portion of the crystals have been ground to the desired resonant frequency, it being., understood that those crystals having acceptable resonant frequencies are removed from, the batch at the end of each grinding operation.

An alternative process of finish grinding the crystals to the desired frequency may be carried out, utilizing the illustrated apparatus, by classifying the crystals according to their resonant frequencies before the grinding is started. More specifically. the crystals are lap ground to have resonant frequencies ranging from l5 to 85 kilocycles less than the particular desired value, and

are then classified in groups according to the resonant frequencies thereof so that the resonant frequencies of the crystals in each group are not more than ten kilocycles apart and the frequency bands of the different groups are nonoverlapping. After the classification ls completed, those crystals that need the most grind ing to raise the resonant frequencies thereof to the desired value are first ground for a predetermined time interval. At the end of this interval, the crystals in the next adjacent ten kilo cycle band are placed in the container il for grinding without removal of the crystals of the i'irst group, and the grinding is continued for an additional predetermined time interval. .The remaining crystals of the batch are successively added `on a group basis in the order of increasing group frequency at. the respective ends of additional predetermined grinding intervals. After those crystals of the group requiring the least grinding have been placed upon the surfaceof the abrasive material Il the grinding operation is continued until all crystals of the batch require additional grinding for an addi-l tional interval of A to l hour in order to raise the resonant frequencies thereof to the particular desired values. At this time the predominate y. portion of the crystals have resonant frequencies `4- different frequency groups upon the abrasive materia 3| is repeated. At the end of the first grind operation, approximately five per cent of the crystals will be found to have the desired resonant frequencies and may be removed from the batch. At the end ofthe second grinding operation` approximately eighty percent of the crystals will be found to have thedesired resonant frequency within permissible tolerances. If the process is repeated a third time on a controlled time basis, another ten percent of the crystals will be found to have the desired relonant frequencies.

From the above explanation it will be understood that as the successive grinding operations are performed, the crystals involved therein approach with increasing nearness the particular desired frequency. In order to prevent over grinding. it is desirable to reduce the grinding rate during the final -grinding operations. This may conveniently be done by progressively decreasing the size of the abrasive material used during the successive grinding operations. Although silicon carbide has been mentioned as a preferred form of grinding media. it will be understood that other abrasive materials, such, for example, as boron carbide, Alundum. tungsten carbide, molybdenum carbide and tantalum carbide may be used. The only apparent limitation upon the type of abrasive material which may be used is that the material from which the piezoelectric crystals are formed be less hard than the abrasive material.

While the invention has been described with particular reference to a specific embodiment thereof, it will be understood that various modifications may be made therein, which are within the true spirit and scope of the invention as denned in the appended claims.

Iclaim:

l. The method of grinding piezoelectric crystals to increase the resonant frequencies thereof to a predetermined desired value. which comprises supporting the crystals upon a mass of loose abrasive material, and oscillating the crystals and. abrasive material to produce relative movement between the surfaces of said crystals and said abrasive material. v i

2. The method of face grinding piezoelectric crystals to increase the resonant frequencies thereof to a predetermined value. which comprises supporting the crystals face downward upon an abrasive surface comprised of loose granular abrasive material, and concurrently oscillating and tilting the crystals and abrasive material to produce relative movement between the faces of said crystals and said abrasive material.

3. The method of face grinding piezoelectric crystals to increase the resonant frequencies thereof to a predetermined value,'which comprises supporting the crystals face downward upon an abrasive surface comprised of loose and abrasive material away from said fiat surface to change the positions of said. crystals relative to said abrasive material, and oscillating said supporting surface to produce relative slidingemovement between the faces of said crystals and said abrasive material. y

5. Apparatus for face grinding piezoelectric crystals to increase the resonant frequencies thereof to a predetermined desired value, comprising means providing a substantially nat and rough supporting surface for a mass of loose abrasive material and crystals. means for intermittently throwing said crystals and abrasive material away from said' flat surface to change the positions of/said crystals relative to said abrasive material, and means for oscillating said surface to produce relative sliding movement between the faces of the crystals and the abrasive material supported thereon.

6. Apparatus for face grinding piezoelectric crystals to increase the resonant frequencies granular abrasive material oscillating the crystals supporting a mass of loose abrasive material upon a rough and substantially flat surface, supporting said crystals face downward upon said abrasive material, intermittently throwing said crystals thereof to a predetermined desired value, comprising a container having a substantially nat, and resilient bottom means providing a surface for supporting a mass of loose abrasive material and crystals. a support, flexible means interconnecting said support and said container to support said container for swinging movement through a predetermined arcuate path, means y carried by said container for intermittently striking said bottom means to throw said crystals away from said surface, thereby to change the positions of said crystals relative to said abrasive material, and means for oscillating said container back and forth along said path, thereby to produce relative sliding movement between the faces of the crystals and the abrasive material supported upon said surface. v

7. Apparatus for face grinding piezoelectric crystals to increase the resonant frequencies thereof to a predetermined desired value, comprising a at resilient member. means movably supporting said member in a substantially horizontal plane so that the upper surface thereof may be utilized to support a mass of loose abrasive material and crystals, means for oscillating said member back and forth through a given path to produce relative sliding movement between the faces of the crystals and the abrasivematerial supportedv thereon, and impacting means for intermittently striking said member in a direction substantially normal to its upper surface, thereby to change the faces of said crystals which are disposed downwardly against said abrasive material.

8. Apparatus for face grinding piezoelectric crystals to increase the resonant frequencies thereof to a predetermined value, comprising a i container having a substantially dat and resilient bottom panel. a support, flexible means interconnecting said support and said container to support said container so that the upper surface of said bottom panel is normally disposed in a substantially horizontal plane. whereby said upper surface may be utilized to support a mass of loose abrasive material and crystals, means for oscillating said container back and forth through an arcuate path to produce relative sliding movement between the faces of said crystals and the abrasive material. and impacting means for intermittently striking said bottom panel in a direction substantially normal to its upper surface. thereby intermittently to change the faces of said crystals which are disposed downwardly against said abrasive material.

HAL F. FRU'IH. 

